1. Field of the Invention
The present invention relates to a capacitive fingerprint sensor, more specifically, a capacitive fingerprint sensor which is able to enhance sensing sensitivity and is resistant to ESD damage or a physical impact.
2. Description of the Related Arts
FIG. 1 through FIG. 4 are equivalent circuit views illustrating a capacitive type fingerprint sensor using a thin film transistor according to some exemplary embodiments of a conventional art.
As illustrated in FIG. 1, the conventional capacitive type fingerprint sensor using the thin film transistor uses an active pixel structure to thereby include a source follower such as a T2 within a pixel, and thus is a sensor which can be driven without a preamplifier by recognizing the change of a capacitance based on fingerprints as the change of a voltage in an ADC (Analog-to-Digital Converter) through a data read-out line. A reset is performed using a VDD voltage within the pixel.
The conventional capacitive fingerprint sensor uses a way in which different voltages from each other are applied to a gate of the T2 one time during one frame depending on a fingerprint by using a scan signal or applying a pulse voltage Vpulse which provides separate capacitive coupling so that different currents from each other can flow through the T2. At this time, when a capacitance by the fingerprint is Cfp and the amplitude of a scan signal is ΔVpulse, the voltage applied to the gate may be expressed as following mathematical formula 1.
                              Δ          ⁢                                          ⁢                      V                          g              ⁢              _              ⁢              T              ⁢              2                                      =                                                                              C                  S                                +                                  C                                      para                    ⁢                    _                    ⁢                    T                    ⁢                    1                                                                                                C                  S                                +                                  C                                      para                    ⁢                    _                    ⁢                    T                    ⁢                    1                                                  +                                  C                                      para                    ⁢                    _                    ⁢                    T                    ⁢                    2                                                  +                                  C                  fp                                                      ⨯            Δ                    ⁢                                          ⁢                      V            pulse                                              {                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                }            
ΔVg-T2 is the voltage applied to the gate of the T2, and the value thereof is changed depending on the capacitance Cfp generated by the fingerprint.
However, in the conventional capacitive fingerprint sensor, when the fingerprint capacitance Cfp is small, namely, a difference between the capacitances based on ridges of the fingerprint is small, ΔVg-T2 (a difference between the voltages applied to the gate of the T2) becomes smaller, thereby reducing fingerprint sensing sensitivity.
The sensing sensitivity is expressed as a dynamic range,
      (          =                        20          ⨯          log                ⁢                  Isignal          Idark                      )    ,and the conventional capacitive fingerprint sensor has a limit to increase a difference between the Isignal and the Idark, thereby generating a limit to enhance the sensing sensitivity.
Thus, in the way of the conventional capacitive fingerprint sensor which senses the fingerprint formed by using the fingers and a protective film of an upper part of the sensor as a dielectric, since the protective film of the upper part of the sensor may not be formed to be thick, the conventional capacitive fingerprint sensor is certainly weak to ESD damage or physical damage.